It is well known that single mode optical fibers in fluoride glass intended for manufacturing high-efficiency optical amplifiers require a ratio between core and cladding diameters of the order of 1:100. Taking into account that the diameter of the cladding of these fibers must correspond with that of a conventional optical fiber for telecommunications (typically 125 .mu.m), for obvious reasons of compatibility, this means that the diameter of the core must be slightly larger than 1 .mu.m (typically, about 1.5 .mu.m).
Current methods for the production of preforms for fluoride glass fibers (e.g. those known in the art as "rotational casting" or "build in casting" and others) do not allow obtaining diameter ratios of the aforementioned order of magnitude, but only ratios of the order of 2-3: 10. Actually, on the one hand it is not possible to form cores whose diameter is less than a few millimeters and, on the other hand, the maximum diameter of the cladding must not exceed about ten millimeters, as larger diameters cause glass stability problems and do not allow drawing the preform.
For this reason, the techniques generally utilized to produce single mode optical fibers in fluoride glass entail, before drawing the preform, "stretching" it and subsequently coating the stretched preform with a tube of the same composition as the cladding. The stretching causes a reduction of the overall diameter of the initial preform (and thus of the core); coating with the tube, which comes to be part of the cladding, allows increasing the ratio of the cladding diameter to the core diameter.
It may be necessary to repeat the stretching and coating operations several times before obtaining the desired diameter ratio. The final preform is then drawn, generally after having been collapsed. An example of this technique is described in the paper "Fabrication of single mode ZBLAN optical fibers", by W. Andrews, D. Coulson and G. Rosman, Journal of Non-Crystalline Solids 140 (1992), pages 281-284.
This technique requires repeated operations at a temperature higher than the glass transition temperature of the material (more particularly, in addition to the manufacturing and drawing of the preform, a heating is required for each stretching of the preform and each coating with a tube), and this gives rise to processes of crystallization or de-vitrification of the glass matrix, thereby worsening the mechanical and optical characteristics of the fiber. The presence of additional interfaces, created by the coating of the stretched preform, also contributes to worsen the mechanical and optical characteristics of the fiber.